1. Field of the Invention
The present invention relates to the field of combustion, and more particularly but not by way of limitation, to a multiple purpose combustion process and apparatus for substantially inhibiting the generation of deleterious constituents such as nitrogen oxides and carbon monoxide in combustion effluents.
2. Discussion of Prior Art
The permissible amount of certain compounds in industrially vented glass is regulated by various governmental agencies, especially in highly populated areas where air quality is adversely affected by the combustion of hydrocarbon fuels. Among such controlled emissions are the oxides of nitrogen, known to produce smog, and carbon monoxide, both of which are pollutants emitted during the combustion of industrial fuels.
In every combustion process where oxygen and nitrogen are present, high flame temperatures result in the fixation of oxides of nitrogen. Such compounds occur in flue gases mainly as nitric oxide (NO), with lesser amounts of nitrogen dioxide (NO.sub.2), nitrous oxide (N.sub.2 O) and other oxides. Since nitric oxide continues to oxidize to nitrogen dioxide in air at ordinary temperatures, the total amounts of nitric oxide, nitrogen dioxide and other oxides of nitrogen in a flue gas effluent are commonly referred to collectively as nitrogen oxides, or NO.sub.x, and expressed as NO.sub.2.
The nitrogen oxides formed in combustion processes include fuel NO.sub.x (resulting from oxidation of nitrogen compounds in certain fuels); prompt NO.sub.x (a baseline of nitrogen oxides promptly formed in normal combustion of hydrocarbon fuels in air); and thermal NO.sub.x (produced from high combustion temperatures in air). It is the generation of this thermal NO.sub.x which the present invention effectively inhibits.
Combustion reactions that produce thermal NO.sub.x also produce carbon monoxide (CO), and attempts to reduce such NO.sub.x production can actually lead to an increase in CO emissions. Early air quality standards recognized the need for the control of NO.sub.x emissions, but largely ignored the CO content of stack gases. When more recent air quality standards placed limits on both NO.sub.x and CO emissions in stack gases, many of the prior art processes and apparatuses designed to reduce NO.sub.x were no longer acceptable solutions to effectively control these deleterious emissions.
As air quality standards have broadened to include CO emission limits, no lesser emphasis has been given to NO.sub.x emission levels. In fact, the limit on the latter has been decreased dramatically. Where in the past many considered NO.sub.x emissions of between about 40 to 60 ppm as being good control (as reflected by prior art developments in this area of technology), much more stringent control of NO.sub.x emissions are now required in many areas of this country. For example, the South Coast Air Quality Management District of California, the regulatory agency over the Los Angeles Basin, has set NO.sub.x emissions at not to exceed 0.03 lbs/MM Btus--roughly 25 parts per million by volume dry--a NO.sub.x level unachieveable by most prior art combustion apparatuses now operating or available, or where achievable, only for a narrow range of operating conditions. These same air quality standards require that CO emissions do not exceed about 100 parts per million.
Over the years, changing air quality standards have lead to considerable prior art efforts to provide apparatuses that remove or prevent the formation of pollutants so that flue gases generated as a result of combustion processes are dischargeable to the atmosphere with minimal deleterious effects on the environment. Generally, these prior art attempts have been toward either preventing the formation of NO.sub.x during the combustion process or controlling NO.sub.x emissions with post combustion treatments. But, as stated, prior art attempts to minimize NO.sub.x emissions have often resulted in the production of unacceptable limits of other deleterious pollutants.
It is known that thermal NO.sub.x formation can be reduced by lowering the flame temperature and delaying combustion, as by injecting an inert gas such as steam into the combustion zone. However, prior art processes and apparatuses which practice inert gas injection to lower flame temperature, generally encounter high carbon monoxide production (above 100 ppmvd) and flame instability. That is, the lower flame temperatures provide higher CO emissions, and the inert gas rates required to effect NO.sub.x reduction also cause the air/fuel/inert gas mixture to fall outside of the flammability limits of the mixture at times, causing flame instability which is manifested as a flame out, a condition which cannot be tolerated in combustion operations.
One prior art teaching in this field is that found in U.S. Pat. No. 4,496,306, issued to Okigami et al. In the Okigami burner assembly primary fuel and air are injected at an inlet end of a furnace where the primary fuel is burned in a first combustion zone. Air is supplied to this zone at a rate required for the combustion of the total fuel. Secondary fuel is injected at a second combustion zone in the furnace at a location spaced downstream from the first combustion zone. The secondary fuel is exposed to random dilution with surrounding combustion products prior to combusting in the furnace with excess oxygen from the first combustion zone.
U.S. Pat. No. 4,095,929, issued to McCartney, teaches a burner assembly for burning a product gas having a low heating value. Recognizing that variations in fuel composition lead to undesirable flame stability under conditions of low load, the burner assembly is provided with an oversized throat, and the fuel and air are each divided into two streams. All of the combustion air is passed through the oversized burner throat as primary and secondary air streams that are both needed at high loads. Primary fuel is supplied through the burner throat by a gas gun, and the remainder portion of the fuel, bypassing the burner throat, is supplied downstream as secondary fuel through an annulus surrounding the throat. Under conditions of low load, both the secondary air through the throat and the secondary fuel are shut off, with the purpose of sustaining adequate turbulence at low loading in order to maintain flame stability. Thus, flame stability rather than NO.sub.x generation controls the design criteria.
As mentioned, inert gas has been employed in combustion processes, and some have employed external flue gas recirculation in an attempt to control the formation of NO.sub.x during combustion of fuels. That is, a portion of the flue gas generated by combustion is collected and mixed with the inlet air fed to the burner. An example of such a process is disclosed in U.S. Pat. No. 4,445,843 issued to Nutcher.
A premix burner which delays the mixing of secondary air with the combustion flame and allows flue gas to mix with the secondary air is taught by U.S. Pat. No. 4,629,413, issued to Micheson et al. A fuel jet eductor entrains primary air to pass a sub-stoichiometric air/fuel mixture to a centrally disposed burner tip, while secondary air is dispensed from an annular space formed about the burner. Small amounts of flue gas are entrained into the fuel rich flame, purportedly providing cooling and dilution of the flame. The patent discusses NO.sub.x emission levels of between about 40 to 120 ppmvd.
Many of the problems inherent in the processes and apparatuses of the prior art for reducing NO.sub.x emissions have been obviated by the burner assembly disclosed in our U.S. Pat. No. 5,044,932 in which a burner tile is disposed about a central fuel nozzle and an air inlet port. Secondary fuel nozzles are disposed peripherally about the burner tile. A barrier member in proximity to the furnace floor forms a flue gas tunnel to collect internal flue gas, and the collected flue gas is passed to the vicinity of the secondary fuel nozzles where a portion is aspirated into the combustion zone by fluid driven eductors through access openings in the burner tile.
Previously known processes and apparatuses are generally capable of reducing NO.sub.x emission levels, but numerous disadvantages or limitations limit the applications for such processes and apparatuses, including the prior art burner assemblies discusses above. Such processes and apparatuses variously fail to provide full emission control; incur flame instability; produce additional emission constituents that are themselves recognized as undesirable; require additional costs, including initial capital outlay and ongoing operating expenses; and many present unacceptable liability exposure.
Processes and apparatuses capable of producing acceptable NO.sub.x and CO emission levels and which overcome the numerous disadvantages and limitations of previously known processes and apparatuses are constantly being sought. It is to such that the present invention is directed.